Rockwell Automation 1734-VHSC24 Very High-Speed Counter Modules User Manual - Manual (Page 16)

Publication 1734-UM003B-EN-P - August 2005
1-6 About the Modules
As the frequency of the incoming pulse train at the Z (Gate/Reset)
terminal increases, the number of sampled pulses from the 5MHz
clock decreases. Since accuracy is related to the number of pulses
received over the sample period, the accuracy decreases with
increasing frequencies at the Gate/Reset terminal. Refer to the
following Scaling table.
Scaling the input frequency through the use of a scalar can lessen the
decrease in accuracy. A scalar value of 1 returns an accurate input
frequency if incoming input pulses have a 50% duty cycle.
Operation of Scalar
In the Period/Rate mode, the scalar lets the incoming pulse train at the
Z Gate/Reset pin be divided by a user-defined number. There is one
scalar value for each counter. Acceptable values for the scalar are 1,
2, 4, 8, 16, 32, 64, and 128. The default value for each scalar is 1.
Note that a 0 scalar is equivalent to a 1.
The product of the Sample Period times the scalar should be less than
6.71 seconds in order to avoid a zero frequency detect indication.
(5 MHz sample time = 200ns; 16,777,216 counts x 200ns x 2 half cycles
of Z = 6.71 seconds)
Relationship Between Sampled Pulses and Input Frequency
Input Frequency at Z Gate/Reset Terminal Sample Pulses for 1/2 Cycle of Z Gate/Reset Pulse
2.5 Hz 1 M
5 Hz 500 k
10 Hz 250 k
20 Hz 125 k
50 Hz 50 k
100 Hz 25 k
200 Hz 12.5
500 Hz 5 k
1k Hz 2.5 k
2 Hz 1.25 k
5 kHz 500
10 kHz 250
20 kHz 125
50 kHz 50
100 kHz 25